Device for reducing flicker defects

ABSTRACT

The invention relates to a device for reducing flicker disturbances in a video signal having a temporal median filter and a filter connected downstream. The temporal median filter serves for compensating for brightness fluctuations in the video signal. The filter connected downstream separates high-frequency signal components from the output signal of the temporal median filter and suppresses high-frequency signal components with a high amplitude.

The invention relates to a device for reducing flicker disturbances in avideo signal having a temporal median filter for compensating forbrightness fluctuations in the video signal.

In historical film material it is often possible to observe large-areaand high-frequency—and thus highly disturbing—flicker effects. Thesebrightness fluctuations that occur from film picture to film picture arepartly due to ageing and environment, but can also occur as a result ofimproper handling in the case of new film material. Brightnessfluctuations which occur from picture to picture can be effectivelyreduced by a temporal median filter. However, as undesired side effects,temporal motion artefacts are introduced into the output signal. Thesaid artefacts can be reduced by a motion detector which switches off orreduces the filtering starting from a certain difference between inputsignal and output signal. Since the motion threshold has to be set tothe value of the flicker amplitude (typically 6%), artefacts that arestill visible occur in the case of fast motion. Such a filter isimplemented e.g. in the MNR11 Noise Reducer from Philips.

The invention is based on the object of providing a device for reducingflicker disturbances which enables the motion artefacts introduced bytemporal median filtering to be largely eliminated.

The object is achieved by means of a device for reducing flickerdisturbances in a video signal of the type mentioned in the introductionby virtue of the fact that the device contains a temporal median filterfor compensating for brightness fluctuations in the video signal and afilter connected downstream for separating a high-frequency signalcomponent from the output signal of the temporal median filter andsuppressing high-frequency signal components with a high amplitude.

Exemplary embodiments of the invention are explained in more detailbelow with reference to the figures, in which:

FIG. 1 shows a temporal median filter,

FIG. 2 shows a filter operating according to the coring technique,

FIG. 3 shows the invention's device for reducing flicker disturbances,and

FIG. 4 shows exemplary time profiles of signals which occur in thedevice according to FIG. 3.

The known temporal median filter illustrated in FIG. 1, which containstwo delay stages 1 and 2 and a median selector 3, reduces brightnessfluctuations that occur from picture to picture. The delay stages 1 and2 delay the picture signal for the duration of a frame. The medianselector 3 usually comprises a processor which combines pixels of theinput signals of the delay stages 1 and 2 and of the output signal ofthe delay stage 2.

However, the temporal median filter (cf. FIG. 1) introduces undesiredtemporal motion artefacts into the output signal of the delay stage 2.In order to largely eliminate these motion artefacts, according to theinvention a temporal median filter is combined with a filter operatingaccording to the coring technique.

FIG. 2 illustrates a known filter operating according to the coringtechnique, which comprises a low-pass filter 4, subtraction circuit 5and 6 and an amplitude filter 7. The input signal of this filteroperating according to the coring technique is fed to the low-passfilter 4, a non-inverting input of the subtraction circuit 5 and anon-inverting input of the subtraction circuit 6. The output signal ofthe low-pass filter 4 is fed to an inverting input of the subtractioncircuit 5, which forwards the signal resulting from the subtraction tothe amplitude filter 7. The amplitude filter 7 suppresses all signalcomponents with a high amplitude. An inverting input of the subtractioncircuit 6 receives the output signal of the amplitude filter 7 andcombines this output signal with the input signal.

With the aid of the filter operating according to the coring technique,the high-frequency component is extracted from an input signal andsubtracted from the input signal for small amplitudes. Thus, a filtering(or noise reduction) takes place only for high-frequency signals with asmall amplitude, as a result of which a loss of relevant information islargely avoided.

FIG. 3 illustrates the device according to the invention having atemporal median filter and having a filter operating according to thecoring technique. The device for suppressing flicker disturbancescontains a temporal median filter, which comprises delay stages 8 and 9and a median selector 10, and subtraction circuits 11 and 12, a furtherdelay stage 13, a horizontally and vertically acting median filter 14and an amplitude filter 15. The median selector 10 receives the inputsignal IN of the device and the output signals FY and FZ of the delaystages 8 and 9. The output signal FY of the delay stage 8, whichreceives the input signal IN, is fed to a non-inverting input of thesubtraction circuit 11 and, via the delay stage 13, to a non-invertinginput of the subtraction circuit 12. The output signal LP of the medianselector 10 is the input signal of an inverting input of the subtractioncircuit 11, which forwards its output signal HP via the horizontally andvertically acting median filter 14 (output signal ME) to the amplitudefilter 15. The output signal CO of the amplitude filter 15 is fed via aninverting input to the subtraction circuit 12, which forms the outputsignal OUT of the device for reducing flicker disturbances.

The temporal median filter selects the mean brightness value over atleast three film pictures and thus suppresses flicker disturbances.However, temporal motion artefacts are introduced in the signal LP. Atemporally median-filtered high-pass signal comprising the flickerdisturbances and the motion artefacts is present at the output of thesubtraction circuit 11. Since the flicker disturbances occur over theentire picture (that is to say over a very large area), they passthrough the horizontally and vertically acting median filter 14unimpeded. The motion artefacts, whose horizontal and vertical size isrestricted to the size of the moving objects, are suppressed by themedian filter 14. In practice, a filter aperture over approximately 9lines and 15 pixels has proved to be sufficient.

In principle, instead of the horizontally and vertically acting medianfilter 14, a linear 2D filter could also be used for suppressing theartefact signal. The median filter 14 is preferred, however, forcompletely eliminating the artefacts having a pulsed profile.

The exemplary time profiles of signals that occur in the deviceaccording to the invention are shown in FIG. 4. FIG. 4 shows theorigination of the artefact signal for a rapidly moving object (thepicture-to-picture displacement is larger than the object width) and fora slowly moving object (the displacement is smaller than the objectwidth). In order to completely suppress the said artefact signal, theaperture of the median filter 14 must be at least twice as large as therapidly moving object. The downstream amplitude filter 15 with a coringcharacteristic curve additionally ensures that residual artefacts with alarge amplitude that possibly occur are suppressed. The thresholds A and−A of the coring characteristic curve are set to the largest flickeramplitude to be expected. The disturbance signal elimination takes placeby subtraction of the output signal (flicker signal) of the amplitudefilter 15 from the delay-equalized input signal (T+1 frame/picture)(output signal of the delay stage 13). T is the delay time of thehorizontally and vertically acting median filter 14. It goes withoutsaying that by using a temporal median filter having more than two delaystages (selection of three pictures), it is possible to extend thesuppression to lower-frequency flicker disturbances. In the 60 Hzstandard, the median filter having two delay stages has the maximumsuppression or extinction at 6.25 Hz flicker frequency, which alreadyyields very good results.

1. Device for reducing large area flicker in a video signal having atemporal median filter for compensating for brightness fluctuation inthe video signal, a median filter connected downstream from saidtemporal median filter for suppressing motion artifacts in an output ofsaid temporal median filter, a filter connected downstream from saidmedian filter for passing high-frequency signal components with a lowamplitude; and a first subtraction circuit for subtracting thehigh-frequency signal components with a low amplitude from a delayedvideo signal, wherein the median filter connected downstream contains asecond subtraction circuit and wherein the filter connected downstreamfrom said median filter comprises an amplitude filter, wherein thesecond subtraction circuit is provided for combining the inverted outputsignal of the temporal median filter and the delayed video signal andthe amplitude filter is provided for suppressing the signal componentswith a low amplitude in the output signal of the second subtractioncircuit, and wherein the first subtraction circuit which supplies theoutput signal from which flicker disturbances have largely beeneliminated, is provided for combining the delayed video signal and theinverted output signal of the amplitude filter.
 2. Device according toclaim 1, wherein a horizontally and vertically acting median filter isconnected between the second subtraction circuit and amplitude filter.3. A method for reducing flicker disturbances in a video signalcomprising the activities of: compensating for brightness fluctuation inthe video signal using a temporal median filter; suppressing motionartifacts in a compensated video signal, wherein suppressing motionartifacts includes combining the inverted output signal of said temporalmedian filter and a delayed video signal and suppressing signalcomponents with a low amplitude in the combined signal; passinghigh-frequency signal components with a low amplitude in a suppressedcompensated video signal to produce a filtered video signal; andsubtracting the high-frequency signal components with a low amplitude ofthe filtered video signal from a delayed video signal for supplying anoutput signal from which flicker disturbances have largely beeneliminated.
 4. A device for reducing large area flicker in a videosignal comprising: a temporal median filter for compensating forbrightness fluctuations in the video signal; a first subtraction circuitfor subtracting the compensated signal from the video signal, therebyproducing a temporally median filtered high-pass signal; a spatialfilter receiving the temporally median filtered high-pass signal; anamplitude filter receiving the spatially filtered signal and suppressingsignals having an amplitude exceeding a threshold; a second subtractioncircuit for subtracting the amplitude-filtered signal from the delayedvideo signal and providing an output signal from which flickerdisturbances have largely been eliminated.
 5. The device of claim 4,wherein the spatial filter comprises a horizontally and verticallyacting median falter or a two-dimensional linear filter.
 6. A method forreducing large area flicker in a video signal comprising the steps of:compensating for brightness fluctuation in the video signal using atemporal median filter; subtracting the compensated signal from thevideo signal, thereby producing a temporally median filtered high-passsignal; passing the temporally median filtered high-pass signal via aspatial filter; passing the spatially filtered signal via an amplitudefilter, suppressing signals having an amplitude exceeding a threshold;subtracting the output signal of the amplitude filter from a delayedvideo signal; and providing an output signal from which flickerdisturbances have largely been eliminated.